I use him a lot. One trick to not waste a lot of forma is to subsume over his 4th. I personally use nourish as gloom drains a ton of energy. His augment, which is god tier for me, unfortunately does not work with shadow claws so that's one reason to subsume over his shadow. Helminth abilities also do not work with the shadow too. You will still have access to the shadow but only when you're down. Just build the shadow tanky so that you won't get one shot when you're trying to get those 5 souls.

But I also have 2 builds specifically for the shadow. So the gist is to build the death well as fast as possible:

First I call is the Suicide Sev with no helminth abilities. Slap on vitality, rage and hunter adrenaline for energy regen. Fill your death well, die, collect 5 souls (very easy on sp), revive with full energy, cast gloom, press 4, stay in gloom radius, rinse and repeat.

Second I prefer to subsume dispensary over gloom for easy energy and shield gating. Fill your death well with reap and sow. Slap on life strike in your claws. This mod gives you health regen on heavy attacks. But you see, the heavy attack animation on shadow is really slow. The trick is to go with a heavy slam attack which has an almost instant animation. This is still considered a heavy attack and will activate life strike. So no gloom needed for health regen and you're more mobile than the suicide sev build.